In magnetic recording disk drives, where the magnetic recording media on the disks is a granular metal alloy, such as a CoPt alloy, the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the recorded magnetic transitions and results in random shifts of the readback signal peaks. Higher media noise leads to higher bit error rates. Thus to obtain higher areal densities in magnetic recording disk drives, it is necessary to decrease the intrinsic media noise, i.e., increase the signal-to-noise ratio (SNR), of the recording media.
The media SNR is to first order proportional to 20 log(N1/2), where N is the number of magnetic grains per unit area in the media and SNR is expressed in units of dB. Accordingly, increases in SNR can be accomplished by increasing N. However, N is limited by the individual grain area A required to maintain the thermal stability of the recorded magnetization. This limitation arises because the energy term protecting against thermal degradation is KV, where K is the anisotropy and V is the volume of an individual magnetic grain. KV must be kept greater than a certain value to assure thermal stability of the recorded magnetizations. Increasing N by merely reducing the grain area A will reduce V since V=At, where t is the grain height (i.e., the thickness of the magnetic recording layer), and this will reduce KV, leading to thermal instability. One approach to prevent this problem is to proportionally increase the anisotropy K as V is decreased. However, this approach is limited by the available magnetic write field produced by the recording head. The magnetic field necessary to write the media (i.e., change the recorded magnetizations) is represented by the short-time or intrinsic coercivity H0of the media, which is proportional to K/M, where M is the grain magnetization or magnetic moment. Therefore, increasing K will increase H0 and may prevent the media from being able to be written by a conventional recording head. In summary, to ensure reliable operation of a magnetic recording disk drive the media must have sufficiently high SNR, sufficiently low H0 to be writable, and sufficiently high KV to be thermally stable.
Improved media SNR can be achieved with “laminated” media. In laminated media, the single magnetic layer is replaced with a laminate of two or more separate magnetic layers that are spaced apart and magnetically decoupled by nonmagnetic spacer layers. This discovery was made by S. E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination”, IEEE Transactions on Magnetics, Vol. 26, No. 5, Sep. 1990, pp. 2706–2709, and patented in U.S. Pat. No. 5,051,288. This approach increases SNR because N is increased, e.g., essentially doubled when the laminated magnetic layer contains two magnetic layers. In this approach the same magnetic alloy composition that was used in the single magnetic layer is used in both magnetic layers of the laminated magnetic layer, so that it is not necessary to use a higher K magnetic alloy material. Thus K remains the same as for the single layer media, i.e., each magnetic layer has an intrinsic coercivity H0 capable of being written by a conventional write head. If each magnetic layer in the laminate is also the same thickness as the single magnetic layer the grain volume V remains the same because the grains in the two magnetic layers are magnetically decoupled by the nonmagnetic spacer layer. Thus SNR is increased without a reduction in KV so that thermal stability is not decreased. However, this laminated media approach to increasing media SNR requires substantially thicker media, e.g., a doubling of the total magnetic layer thickness to increase SNR by approximately 3 dB. However, a different problem arises by doubling the thickness of the magnetic layer, namely a degradation in overwrite (OW), which is a measure of how well a second signal can be recorded over a previously written signal. Low OW is undesirable because it means that a larger amount of the original signal remains after it is overwritten by the second signal. Even though the magnetic material in the laminated layer is capable of being written by a conventional write head, low OW occurs in laminated media because the write field decreases with distance from the write head and thus the strength of the write field is less at the bottom magnetic layer than at the top magnetic layer.
Media SNR can also be improved by thermally-assisted magnetic recording (TAMR), wherein a high K magnetic recording layer is heated locally during writing to near the Curie temperature of the magnetic material. In this approach the single magnetic layer having a KV value is replaced with a single magnetic layer of a different alloy composition. This different alloy has smaller grains to thereby increase N, but also a higher anisotropy K. Because the higher K material has a higher H0, it can not be written by a conventional write head and thus the magnetic material must be heated to lower the intrinsic coercivity enough for writing to occur. Several approaches for heating the high K media in TAMR have been proposed, including use of a laser beam or ultraviolet lamp to do the localized heating, as described in “Data Recording at Ultra High Density”, IBM Technical Disclosure Bulletin, Vol. 39, No. 7, July 1996, p. 237; “Thermally-Assisted Magnetic Recording”, IBM Technical Disclosure Bulletin, Vol. 40, No. 10, October 1997, p. 65; and U.S. Pat. No. 5,583,727. A read/write head for use in a TAMR system is described in U.S. Pat. No. 5,986,978, wherein a special optical channel is fabricated adjacent to the pole or within the gap of a write head for directing laser light or heat down the channel. U.S. Pat. No. 6,493,183 describes a TAMR disk drive wherein the thin film inductive write head includes an electrically resistive heater located in the write gap between the pole tips of the write head for locally heating the high K magnetic recording layer.
What is needed is a magnetic recording disk drive with thermally stable, high SNR media that has high OW.